Measuring Gravity of Beer: Hydrometers vs Refractometers

Measuring gravity of beer is one of the simplest ways to monitor fermentation, estimate alcohol content and understand how your brew is progressing.

You may have seen the initials “OG” and “FG” on a recipe, but what do these mean? Original gangster? Final goal? Funky gorilla?

These are references to specific gravity: in short, a measurement of the density of a liquid. On this scale, 1.000 is water. Anything thicker than water (beer, blood, wort, etc.) will read higher.

Brewers typically use tools like hydrometers and refractometers to measure gravity throughout the brewing process.

In this guide, we’ll explain how to measure specific gravity of beer, the difference between a hydrometer and refractometer, and when to use each tool in homebrewing.

What Is Specific Gravity in Beer?

Specific gravity is a measurement of liquid density compared to water. In brewing, it tells us how much sugar is dissolved in the wort or beer.

When we make beer, sugars from malt dissolve into water, making the liquid denser than water alone. During fermentation, yeast consumes these sugars and converts them into alcohol and carbon dioxide. As the sugars are used up, the gravity reading gradually drops.

Because of this, measuring gravity gives brewers a reliable way to monitor fermentation and understand what is happening inside the fermenter.

These gravity readings are usually referred to as Original Gravity (OG) and Final Gravity (FG).

What Is Original Gravity (OG)?

Original Gravity (OG) is the gravity reading taken before fermentation begins. This tells you how much sugar is present in the wort before yeast is added and gives an indication of potential alcohol content.

What Is Final Gravity (FG)?

Final Gravity (FG) is the gravity reading once fermentation is complete. As yeast consumes sugars during fermentation, the gravity drops from the OG down to the FG.

FG can help indicate:

• whether fermentation has finished
• approximate alcohol content
• how dry or full-bodied the beer may taste

Calculating Alcohol Content From Gravity Readings

As well as showing the progress of fermentation, measuring gravity also lets brewers estimate the alcohol content of their beer.

When yeast consumes sugars during fermentation, it produces carbon dioxide (the bubbles through your airlock) and ethanol (alcohol). By measuring how much the gravity drops between your Original Gravity (OG) and Final Gravity (FG), you can work out a rough ABV for the finished beer.

Recipe OG and FG figures should always be treated as targets rather than exact numbers. Depending on your equipment, mash efficiency, yeast performance and fermentation conditions, your readings may end up slightly different — which is why it’s always worth taking your own measurements throughout the brewing process.

To estimate ABV, you’ll need:

• your Original Gravity (OG)

• your Final Gravity (FG)

You can then use an ABV calculator to estimate the alcohol content based on the difference between the two readings.

This won’t give a laboratory-grade alcohol measurement (true ABV calculations are more complex because alcohol is less dense than water), but for homebrewing purposes it’s more than accurate enough to tell whether your latest brew is a light session beer or something with a bit more kick.

(While you’re checking out our calculators, why not use your gravity readings to estimate the calories in your brew?)

Hydrometer vs Refractometer: What’s the Difference?

When measuring gravity of beer, the main difference between a hydrometer and refractometer is the reading method. A hydrometer measures liquid density by floating in a sample, while a refractometer measures how light bends through a small amount of wort.

Both are useful measurement tools, but they suit different points in the brewing process.

Why Use a Hydrometer?

Hydrometers are generally considered the most accurate option for fermentation readings because they can be read directly even once alcohol is present in the beer.

Most brewers draw a sample into a trial jar first rather than placing the hydrometer directly into the fermenter. This makes readings easier to take, avoids contamination risks from opening the fermenter repeatedly, and helps prevent krausen or hop debris affecting the reading.

Hydrometers are especially useful for:

• Original gravity (OG) readings
• Final gravity (FG) readings
• Monitoring fermentation progress
• Estimating alcohol content

Which Tool Should You Use?

Many brewers use both.

A refractometer gives you quick and convenient readings during the mash and boil, while a hydrometer is usually preferred once fermentation starts and for taking final gravity readings.

Using both together gives you a more complete picture of your brewing process, from mash efficiency through to fermentation and finished beer.

Using a Hydrometer to Measure Beer Gravity

A hydrometer is one of the most common tools used for measuring gravity of beer. It measures specific gravity based on how high or low it floats in liquid: the denser the wort or beer, the higher the hydrometer will sit.

A hydrometer is often the preferred measurement tool for fermentation monitoring, taking an original gravity measurement and checking final gravity once alcohol is present in the beer.

If you’re learning how to read a hydrometer, the process itself is straightforward:

• Draw a sample into a clean trial jar

• Lower the hydrometer into the liquid

• Let it settle until it stops bobbing

• Read the scale where the liquid meets the stem

In a narrow trial jar, you may notice a slight curve where the liquid meets the glass stem. This is called the meniscus. For the most accurate reading method, try to ignore the curve and read at the flat part of the waterline.

One thing to bear in mind is hydrometer temperature calibration. Most traditional hydrometers are calibrated to 20°C, though it’s worth checking the original packaging or tube to confirm this. If the wort or beer is significantly warmer or cooler, the reading may need correcting for accuracy. Fortunately, we have a calculator to help:

Hydrometer correction calculator

Glass hydrometers are reliable and easy to use, but they are also fragile. Because they’re weighted to float, they have an unfortunate tendency to roll off worktops and smash just when you need them. Keeping yours stored in the plastic tube it came in is usually a good idea.

Digital Hydrometers

If you want frequent gravity readings (and the fun of automated data collection), a digital hydrometer is a good upgrade.

Digital hydrometers work on the same principle as traditional hydrometers, calculating liquid density based on how they float in the beer. The difference is that they combine this with wireless technology such as Bluetooth or WiFi to provide real-time gravity and temperature readings throughout fermentation.

Most digital hydrometers are designed to stay inside the fermenter during fermentation, allowing you to monitor changes without repeatedly drawing samples or opening the fermenter.

Because they sit directly in the beer, they can often provide a more accurate picture of wort temperature than an external thermometer attached to the outside of the fermenter.

One thing to bear in mind is that krausen, floating hop matter and dry hop additions can sometimes affect accuracy slightly. For this reason, many brewers use both a traditional hydrometer and a digital hydrometer together: using a glass hydrometer for precise OG and FG readings, then using the digital hydrometer to monitor fermentation activity in real time.

Using a Refractometer to Measure Beer Gravity

A refractometer is another useful measurement tool for checking the gravity of wort, especially during the mash and boil. Instead of floating in liquid like a hydrometer, a refractometer works by passing light through a very small sample.

Sugar refracts light differently from pure water, so by measuring the level of refraction, the tool can estimate sugar content. Many refractometers use the Brix scale, which is commonly used to show sugar concentration.

How to Use a Refractometer

To use a refractometer, take a small sample from your wort. You only need a few millilitres, so a pipette or syringe is ideal.

Place a droplet of wort onto the glass lens, close the lid, then hold the refractometer up to the light. Look through the viewing end and adjust the lens until the scale comes into focus. A line will run across the scale at the gravity reading for your sample.

The main benefit is ease of use. Because you only need a tiny sample, you can take quick readings throughout the brewing process without losing much wort.

Refractometer Readings During Fermentation

A refractometer can be used after fermentation has started, but the reading needs correcting. Alcohol affects how light refracts through the beer, so mid-fermentation and final gravity readings will not be accurate without a correction factor.

For this reason, we find it’s generally more accurate to use a hydrometer for fermentation readings and keep the refractometer for mash gravity, pre-boil checks and other hot-side measurements.

FAQs: Hydrometers & Refractometers

Hydrometers, Refractometers & Brewing Tools

Once you start measuring gravity regularly, a few bits of kit quickly become staples on brew day. A decent hydrometer or refractometer makes life much easier, but things like trial jars, syringes and fermentation equipment all help you get cleaner readings and a clearer picture of what your beer is doing throughout fermentation.

Useful equipment and related guides:

• Testing & Measuring Equipment –  hydrometers, refractometers, thermometers and other essential brewing measurement tools
• Brewing Calculators – useful for ABV calculations and hydrometer temperature correction
• Fermentation Guides –useful if you’re tracking gravity changes during fermentation or trying to diagnose stalled fermentations
• Fermentation Equipment –fermenters, sample taps, temperature control and other kit that makes gravity readings and fermentation monitoring much easier